Thermal protector

ABSTRACT

A thermal protector comprising two generally U-shaped strips of bimetal nested one within the other. The first bimetal strip has the layer with the lower coefficient of thermal expansion on the inner surface thereof and has a fixed end and a free end. The second strip of bimetal is nested within the first strip and has the layer with the higher coefficient of thermal expansion on the inner surface thereof with one end secured to a first electrical terminal. The second strip has a thermal force somewhat less than that of the first strip. The free end of the second strip extends outwardly beyond the free end of the first strip and is in physical contact therewith but electrically insulated therefrom. A first electrical contact is secured to a second electrical terminal. A second electrical contact is carried on the free end of the second bimetal strip and projects from the lower expansion layer thereof and is movable toward and away from engagement with said first electrical contact. The nested U-shaped strips are positioned relative to the first contact so that the second contact is in engagement therewith at temperatures below a preselected level. Upon current flow through the second strip exceeding a level that will heat the strips above the preselected temperature level, the first strip will flex to cause initial separation of the contacts and the resulting termination of current flow through the second strip will cause it to cool and flex rapidly to increase the contact separation thereby to effect rapid contact separation under overtemperature conditions of the thermal protector.

United States Patent [191 Moksu THERMAL PROTECTOR [75] Inventor: Walter H. Moksu, Attleboro, Mass.

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex,

22 Filed: Dec. 14,1973

21 Appl. No.: 425,034

Primary Examiner-I-Iarold Broome Attorney, Agent, or FirmHarold Levine; John A. Haug; James P. McAndrews [5 7] ABSTRACT A thermal protector comprising two generally U- shaped stripsof bimetal nested one within the other.

1 1 Sept. 3, 1974 The first bimetal strip has the layer with the lower coefficient of thermal expansion on the inner surface thereof and has a fixed end and a free end. The second strip of bimetal is nested within the first strip and has the layer with the higher coefficient of thermal expansion on the inner surface thereof with one end secured to a first electrical terminal. The second strip has a thermal force somewhat less than that of the first strip. The free end of the second strip extends outwardly beyond the free end of the first strip and is in physical contact therewith but electrically insulated therefrom. A first electrical contact is secured to a second electrical terminal. A second electrical contact is carried on the free end of the second bimetal strip and projects from the lower expansion layer thereof and is movable toward and away from engagement with said first electrical contact. The nested U-shaped strips are positioned relative to the first contact so that the second contact is in engagement therewith at temperatures below a preselected level. Upon current flow through the second strip exceeding a level that will heat the strips above the preselected temperature level, the first strip will flex to cause initial separation of the contacts and the resulting termination of current flow through the second strip will cause it to cool and flex rapidly to increase the contact separation thereby to effect rapid contact separation under overtemperature conditions of the thermal protector.

15 Claims, 6 Drawing Figures III/II/ll I/l VII/Ill 1 f SHEEI 2 OF 2 PAIENIEDSEP 31924 FIG.-6

THERMAL PROTECTOR BACKGROUND OF THE INVENTION This invention relates to a thermal protector for an electric motor, solenoid, transformer or other electrical apparatus, and more particularly to a thermal protector for low ampere miniature motors with ratings in the order of 1.5 to 4 amps.

Thermal protectors of this invention are especially adapted for interrupting current to the windings of a miniature motor or the like to protect it from damaging overtemperature conditions upon the occurrence of an instantaneous massive overload such as caused by a locked rotor, or due to prolonged operation of the motor under a load somewhat above its maximum rated load, i.e., one which exceeds its ultimate trip current.

Thermal protectors, extensively used for protecting most sizes of electric motors and the like, have not beeen satisfactory for use in miniature motors. Typically, reliably operating protectors could not be made which were small enough so as to be readily incorporated in the windings or positioned in the necessary close heat-exchange relationship with the windings of miniature motors and other small electrical apparatus. However, due to the importance of reliable operation ate at a desired or predetermined elevated temperaand protection of precision miniature electric motors, I

it is highly desirable to avoid damage to these motors by overtemperature operation such as will otherwise result from high short term or lesser long term overcurrent conditions.

Earlier protectors have utilized a cantilevered dished bimetal strip contact arm responsive to an elevated temperature to move a contact mounted on the free end thereof into a circuit-breaking position and thus open the circuit to the motor. To protect against locked rotor conditions the strip must almost instantly selfheat from the flow of current through the strip. However, even the highest resistivity composite metal materials that can be used for these devices has insufficient resistance in the short lengths as limited by the miniature size requirements to heat rapidly enough.

Small thermal protectors with U-shaped bimetal contact arm strips have also been used so as to provide increased contact arm length and thus increased resistance. Such U-shaped arms having one portion of a higher flexivity bimetal with its high expansion layer on the outer surface and having an extension of a higher resistance bimetal with its higher expansion layer on the inner surface of the U have also been used. However, again such protectors do notfrespond rapidly enough to reliably protect against locked rotor conditions as well as protecting against overtemperature due to ultimate trip current conditions.

SUMMARY OF THE INVENTION ture; the provision of such a protector which is of small size and yet has the proper electrical oversurface clearances as required by generally recognized safety codes, such as established by Underwriters Laboratories; the provision of such a protector which does not require a separate heater but has rapid response to locked rotortype overloads; the provision of such a protector which is economical to manufacture, has a long service life, and which is reliable in operation. Other objects and features will be in part apparent and in part pointed out hereinafter.

Briefly, a thermal protector of this invention comprises two generally U-shaped strips of bimetal nested one within the other. The first strip has its layer with the lower coefficient of thermal expansion on the inner surface thereof and has a fixed end and a free end. The second has its layer with the higher coefficient of thermal expansion on the inner surface thereof with one end secured to a first electrical terminal. The second strip has a thermal force somewhat less than that of the first strip, and the free end of the second strip extends outwardly beyond the free end of the first strip and in physical contact therewith but electrically insulated therefrom. A first electrical contact is secured to a second electrical terminal and a second electrical contact is carried on the free end of the second bimetal strip and projects from the lower expansion layer thereof and is movable toward and away from engagement with the first electrical contact. The nested U-shaped strips are positioned relative to the first contact so that the second contact is in engagement therewith at temperatures below a preselected level whereby upon current flow between the terminals and through the second strip exceeding a level that will heat the strips above the preselected temperature level, the first strip will flex to cause initial separation of the contacts and the resulting termination of current flow through the second strip will cause it to cool and flex rapdily to increase the contact separation thereby to effect rapid contact separation under overtemperature conditions of the thermal protector.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical longitudinal cross section on line 1-1 of FIG. 2 of a thermal protector of this invention illustrating a pair of nested U-shaped bimetal strips with the thicknesses of the strips being exaggerated for clarity and with the strips shown in their normally closed position;

FIG. 2 is a plan view of the protector of FIG. 1;

FIG. 3 is a semidiagrammatic view illustrating the position of the U-shaped strips upon breaking of the contacts when the protector is subjected to an overtemperature condition;

FIG. 4 is a view similar to FIG. 3 illustrating the position of the U-shaped strips under normal operating conditions;

FIG. 5 is a view similar to FIG. 1 illustrating an alternative thermal protector embodiment of this invention; and I FIG. 6 is a view similar to FIG. 1 illustrating yet another thermal protector embodiment of the invention.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, a first embodiment of a thermal protector of this invention is indicated in its entirety at 1. This protector is particularly useful for protecting miniature electric motors, solenoids, transformers or other electrical apparatus having low (e.g., 1.2-6 amp.) current ratings from overtemperature conditions (either instantaneous or longterm) which may damage the electrical apparatus.

Protector 1 includes a generally U-shaped outer strip, as indicated generally at 3, of bimetal material (this strip also being referred to as a first strip) having a first metallic layer and a second metallic layer 7 bonded together, the first layer 5 having a lower coefficient of thermal expansion than that of the second layer 7. Thus the lower expansion layer or side 5 is disposed on the inner"surface of U-shaped strip 3. One end of the outer U-shaped strip is fixed, as indicated at 9, by welding to the inner surface of a conductive case or housing 10. Its other end 11 is free. A second or inner U-shaped strip generally indicated at 13 is nested within the first or outer strip. The inner strip is also made of bimetal and has its higher expansion side or layer 15 disposed on the inner surface of the second strip and has its second or lower expansion side 17 disposed on the outer surface thereof.

One end of an inner U-shaped strip, as indicated at 19, is commonly secured at 9 as by spot welding to housing 10. This case or housing constitutes a first electrical terminal. As indicated at 23, a thin layer of. insulation is provided between the inner end outer U- shaped strips except at the fixed ends thereof, so that U-shaped strip 13 (FIGS. 1-4) is in outside-face-toinside-face relation with the insulated inside surface of strip 3 along a portion of the inner strip. A first electrical contact 25 is secured to a conductive cover 27 for housing 10, this cover constituting a second electrical terminal. A second contact 29 is carried on the free end of the inner strip 13 which extends beyond the free end of outer strip 3. Upon thermal deflection of the inner U-shaped strip, contact 29 moves toward and away from contact 25.

More particularly, inner and outer U-shaped bimetal strips 3 and 13, respectively, are preferably of conventional thermostatic bimetal material, such as that made and sold under the trademark TRUFLEX by Texas Instruments Incorporated. The outer U-shaped strip 3 is shown to be somewhat thicker than the inner strip 13, preferably having a greater thermal flexivity than the inner strip and, at the same temperature, the outer U- shaped strip exerts greater thermal force than the inner U-shaped strip. Thermal flexivity is defined by the American Society for Testing Materials (ASTM) as the change of curvature of the longitudinal center line of the specimen per unit temperature change for unit thickness. Thus, flexivity is a measure of how much a thermostat bimetal strip moves or deflects with av given change of temperature. The higher the thermal fiex- 6 ivity, the greater the movement. More specifically, thermal flexivity is given by the following formula:

F z),-( 1)l z" 1),

where T abd T temperature, F.,

t= thickness, in.,

B movement, in.

L active length of strip, in.

Thermal force for a U-shaped strip (i.e., the force required to move an unrestrained strip subjected to a temperature difference back to its orginal position) is given by the formula:

P force, ounces E modulus of elasticity W width, inches The inner strip 13 has a relatively high electrical resistance (e.g., 870 ohms/circular mil foot). Additionally, strip l3 preferably has a lower thermal fiexivity than outer strip 3, and it exerts less thermal force at a given temperature. Exemplary thicknesses for thermal element strips 3 and 13 are 12 mils and 5 mils, respectively. A typical width for the strips is .09 inch. It will be understood, however, that the inner element 13 need not be thinner and could be narrower or even wider than outer element 3. Also, the thermal flexivities and even the thicknesses of the strips may be the same as long as the thermal force of the outer strip exceeds that of the inner strip so that with current flow through the inner strip exceeding a level that will heat both the inner and the outer strips above a preselected temperature level (e.g., C.) the outer strip will flex and overcome the thermal force of the inner strip thereby to cause initial separation of contacts 25 and 29 and terminate the flow of current through the inner strip. This resulting termination of current flow in the inner strip causes it to cool and, because its high expansion side 17 is located on the opposite side of the strip from contact 25, cooling of the inner strip causes it to flex rapidly away from contact 25 so as to accelerate contact separation.

A button or boss 33 is formed on the inner surface of the outer U-shaped strip adjacent the free end 11 thereof for engagement with the outer face of the inner U-shaped strip 13 thereby to provide an insulated abutment between the two U-shaped strips and prevent any current from flowing in outer strips 3. Electrical insulation 23 preferably is provided between the outer and inner strips 3 and 13 to further insure the outer strip is electrically insulated from the inner strip. Insulation layer may, for example, be a thin (e.g., 2-3 mils) coating of a polyimide synthetic resin, such as sold by E. I. Du Pont DeNemours and Company under the trademark KAPTON, formed on the low expansion side 5 of the outer strip by electrodeposition processes. Alternatively, this insulating layer may be a coating of a fluorocarbon resin, such as manufactured by Du Pont and sold under the trademark TEFLON, and applied to the strip by means of spray coating or other similar process. The insulative coating is applied to the entire inner surface of strip 3 (i.e., to its low expansion side 7) except in the area of fixed end 9. It will be understood that the insulation layermay be applied to the outer surface of 0 strip 13. As best shown in FIG. 1, ends 9 and 19 of the outer and inner U-shaped strips 3 and 13, respectively,

are secured to one another and to conductive housing 10 by means of spot welds 35 thus providing a conductive path between housing 10 and inner strip 13. It will be understood that the ends 11 and 19 of strips 3 and 13 may first be welded to one another to form a subassembly which may then be welded as a unit to housing 10. Thus, predetermined dimensions of the strips may readily be maintained.

Housing and cover 27 each have an integrally formed respective terminal lug 36 and 37 which may be readily crimp-connected to respective conductors or wires 38 and 39 thereby to serially interconnect protector 1 in a power lead to the motor. Cover 27 is electrically insulated from housing 10 by means of a layer of flexible sheet insulation 41 wrapped about the cover. Housing 10 includes integrally formed end flanges 43 (see FIG. 1) at the sides of the housing bent around the edges of cover 27 and underlying the outer side margins of the cover whereby upon crimping of these end flanges, the cover and insulation 41 are positively secured in place as shown in FIG. 1. Thus, U-shaped strips 3 and 13 are sealed within housing 10 and cover 27. It will be particularly noted that the overall size of housing 21 may be relatively small (e.g., approximately /2X%X% inches) thus permitting protector 1 to be positioned within the windings of a miniature motor. With the protector so installed in the windings it accurately and with rapid response senses overtemperature conditions in the windings.

As indicated at 45, a portion of housing 10 is stepped inwardly toward cover 27, with the inner and outer U- shaped strips 13 and 3 being spot welded, as indicated at 35, to the inner face of this stepped down portion. Cover 10 is preferably made of a relatively light gauge metal and the stepped down portion 45 may be deformed toward cover 27 by squeezing thereby to move U-shaped strips 3 and 13 toward cover 27 to thus increase the closing force on contact 29 against 25 and to thus vary the temperature of the U-shaped strips at which the contacts break. Thus, the protector 1 of this invention may be accurately and conveniently calibrated to open at a predetermined elevated temperature.

Operation is as follows: With the protector l of this invention serially connected in a power lead to a miniature motor and positioned in or in good close heatexchange relationship with the motor windings and calibrated to open at a predetermined elevated temperature (e.g., 150C), current flows between the first conductor 38, through housing 10, the inner strip 13, engaged contacts 29 and 25, cover 27 and conductor 39. Due to the relatively high electrical resistivity of inner strip 13, the current flowing therethrough resistively heats the inner strip as a function of the flow of current. With the high expansion side of inner strip 13 disposed on its inner side, heating of the inner strip causes its free end and contact 29 carried thereby to deflect somewhat toward contact 25 thereby to insure and maintain positive engagement with and good contact pressure against contact 25 (see FIG. 4). Outer strip 3 is conductively heated by the heat generated in strip 13 and by ambient temperature conditions to a steady state operating temperature. As strip 13 is nested within and in excellent heat-exchange relationship with strip 3 there is rapid heat transfer therebetween.

Upon protector 1 being subjected to an overtemperature condition, such as may be caused by excessive current flowing through strip 13, strip 3 will be heated above its predetermined temperature level and the thermal deflection and thermal force generated by the outer strip overcomes the lesser opposing thermal force of the inner strip 13 and thus causes contacts 25 and 29 to break. Immediately upon breaking of the contacts,

current ceases to flow through inner strip 13 and consequently heat generation in the inner strip immediately ceases. The inner strip will rapidly cool which causes the free end of the inner strip and contact 29 to move rapidly away from contact 25 to provide an accelerated contact separation. This rapid separation of the contacts inhibits arcing of the contacts upon breaking. Upon cooling of outer strip 3, its free end 11 will move toward cover 27 and thus permits the resilient spring action of inner strip 13 to move contact 25 back into engagement with contact 25 thereby to again complete the circuit. Upon remaking of the circuit current flowing through the inner strip rapidly heats it and causes some deflection of the free end of the inner strip and contact 29 toward contact 25. This insures good contact pressures. Assuming the overload condition continues to remain, contacts 25 and 29 will continue to make and break in definite on and off cylces and maintain the winding temperature of the motor below a damaging temperature. The relatively long length of the U-shaped strips 3 and 13 results in good thermal action and deflection and the length of inner strip 13 provides a relatively high resistance path for current flow insuring good self-heating characteristics and heat generation.

Referring now to FIG. 5, a second embodiment of a thermal protector of this invention is indicated in its entirety at 47, this second embodiment being similar in many respects to protector 1 with corresponding parts having corresponding reference characters and with corresponding reference characters for protectors 47 having the postscript a. More particularly, protector 47 has an outer U-shaped bimetal strip 3a and an inner U- shaped bimetal strip 13a with a layer of insulative material 23a, such as a polyimide coating heretofore described, interposed between the inner and outer U- shaped strips thereby to electrically insulate the outer strip from the inner strip. In this instance layer 23a is applied to the outer surface of strip 13a. Thus the outer surface of the inner strip and the inner surface of the outer strip engage the insulation layer 23a for substantially the length of the outer strip. The lower or fixed ends of both the inner and outer U-shaped strips are commonly secured or fixed by welding or the like to a dimple 49 formed on cover 27a, these strips being secured to the cover so that inner strip 13a is in good electrical contact with the cover. Welding slug 50 provides for subassembly of 3a and 15a, the secured to cover 49 for the electrical connection and heat sink. Free end 15a of the inner strip projects outwardly somewhat beyond free end of the outer strip and has a wire contact 29a secured to its outer face, this wire contact corresponding to the second contact 29 heretofore described. A stationary or fixed wire contact 25a is secured, as by welding, to the inner face of housing 10a and is disposed generally at right angles to contact 29a.

As heretofore described, strip 3a has higher thermal force than strip 13a so that at the predetermined temperature level, the thermal force of strip 3a overcomes the thermal force of strip 13a and causes the contacts to break.

Briefly, protector 47 operates in a manner generally similar to protector l heretofore described. with contacts 25a and 29a closed (as shown in FIG. 5) and with the thermal protector serially connected in a circuit, current flows through housing 10a, contact 25a,

contact 29a, inner U-shaped strip 13a and through cover 17a. The flow of current through inner strip 13a causes resistive heating thereof and because its high expansion side a faces inwardly of the U-shaped strip, heating of the strip increases the contact pressure between contacts 2 9a and a. The outer U-shaped strip 3a is conductively heated bystrip 13a and because its high expansion side 7a is disposed outwardly of the strip, increase in temperature of the outer strip causes its free end 11a and control 29a to move away from fixed contact 25a. Upon the outer strip attaining a predetermined overtemperature level, the higher thermal force of the outer strip causes contacts 29a and 25a to break thereby interrupting the current flowing through the inner strip and thus resulting in rapid cooling of the inner strip. This rapid cooling causes the free end of the inner strip to thermally deflect away from stationary contact 25a thereby rapidly moving contact 29a away from the stationary contact in a manner as heretofore described. Upon cooling of the inner and outer strips, the contacts 25a and 29a will again move into engage ment with one another to complete the circuit.

A third embodiment, shown in FIG. 6, is indicated at 52. Reference characters for protector 52 which refer to parts corresponding to parts in protector I bear the postscript b. This embodiment is particularly advantageous for providing a hermetically sealed protector and includes first and second terminal pins 53, 54 mounted in spaced apart relation by a glass header 55. Protector 52 has an outer U-shaped bimetal strip 3b and an inner U-shaped bimetal strip 13b with a layer of insulation material 23b, such as a polyimide coating heretofore described, interposed between the inner and outer U- shaped strips thereby to electrically insulate the outer strip from the inner strip. In this instance layer 23b is applied to the inner surface of strip 3b. U-shaped strips are mounted on the free end 56 of pin 53 as by welding thereto. It will be noted that end 56 of pin 53 is bent slightly to provide space for movement of the U-shaped strips. A stationary contact 57 is mounted on the free distal end 58 of pin 54 so that contact 29b mounted on the free end of U-shaped strip 13b can move into and out of engagement therewith. The thermal protector assembly comprising glas header 55, pins 53, 54 and U-shaped strips 3b and 13b are received in a glass envelope 60. Attachment of envelope 60 to glass header 55 is effective to provide a hermetic seal for protector 52 so that it can be used in environments requiring such seals. Since the operation of protector 52 is the same as that of protector 1 its description will not be repeated. It is of course within the purview of the invention to dispose U-shaped strips 3b and 13b so that contact ends are adjacent to the glass header and the bight portion of the U-shaped strips are adjacent the closed end of the envelope, if so desired.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A thermal protector comprising:

a first generally U-shaped strip of bimetal having the layer with the lower coefficient of thermal expansion on the inner surface thereof and having a fixed end and a free end;

a second generally U-shaped strip of bimetal nested within the first strip and having the layer with the higher coefficient of thermal expansion of the inner surface thereof with one end secured to a first electrical terminal, said second strip having a thermal force somewhat less than that of the first strip, the free end of the second strip extending outwardly beyond the free end of the first strip and in physical contact therewith but electrically insulated therefrom;

a first electrical contact secured to a second electrical terminal; and

a second electrical contact carried on the free end of the second bimetal strip and projecting from the lower expansion layer thereof and movable toward and away from engagement with said first electrical contact, said nested U-shaped strips being positioned relative to the first contact so that the second contact is in engagement therewith at temperatures below a preselected level whereby upon current flow between said terminals and through said second strip exceeding a level that will heat the strips above the preselected temperature level the first strip will flex to cause initial separation of the contacts and the resulting termination of current flow through said second strip will cause it to cool and flex rapidly to increase the contact separation thereby to effect rapid contact separation under overtemperature conditions of the thermal protector.

2. A protector as set forth in claim 1 wherein the second strip has a higher resistivity than that of said first strip.

3. A protector as set forth in claim 1 wherein a layer of electrical insulation is interposed between the inner surface of said first U-shaped strip and the outer surface of the second strip.

4. A protector as set forth in claim 3 wherein the insulation layer comprises a coating of a polyimide synthetic resin.

5. A protector as set forth in claim 3 wherein said insulative layer is applied to the inner face of said first strip and said second U-shaped strip engages said insulative layer at least partially along the length of said second U-shaped strip thereby to enhance heat transfer between said strips.

6. A protector as set forth in claim 3 wherein the outer surface of the second U-shaped strip and the inner layer of the first U-shaped strip both engage said insulative layer for substantially the length of said first U-shaped strip.

7. A protector as set forth in claim l wherein there is an insulating abutment on the inner surface of the free end of the first strip and the outer surface of the free end of the second strip bears thereagainst.

8. A protector as set forth in claim 2 wherein the flexivity of the first strip substantially exceeds that of the second strip.

9. A protector as set forth in claim 1 further comprising a conductive housing, said one end of said first U- shaped strip being fixed relative to the housing and said one end of said second U-shaped strip being secured to and in electrical contact with said housing, said housing constituting said first terminal.

10. A protector as set forth in claim 9 further comprising a conductive cover closing said housing and insulative means for electrically insulating said housing from said cover, said cover having said first contact secured thereto and constituting said second terminal.

11. A protector as set forth in claim 9 wherein said housing has a deformable portion for adjusting the positioning of the two nested U-shaped strips and first contact relative to the second contact variably to bias the contacts together at temperatures below said preselected level thereby'to calibrate the protector to open the contacts at a desired preselected temperature.

12. A protector as set forth in claim 10 wherein said housing and said cover each have means for ready connection to an electrical circuit.

13. A protector as set forth in claim 11 wherein the fixed ends of said U-shaped strips are commonly welded to said housing at the same point and wherein said insulative means comprises a layer of polyimide synthetic resin applied to the inner surface of said first strip to electrically insulate saidrfirst strip from said second strip, said first strip being clear of said polyimide coating in the area where said strips are welded together.

14. A protector as set forth in claim 3 wherein the insulation layer comprises high temperature resin.

15. A protector as set forth in claim 1 further comprising a terminal pin, a glass envelope, into which said pin extends, said one end of said first U-shaped strip being fixed relative to the envelope and said one end of said second U-shaped strip being secured to and in electrical contact with said pin, said pin constituting said first terminal. 

1. A thermal protector comprising: a first generally U-shaped strip of bimetal having the layer with the lower coefficient of thermal expansion on the inner surface thereof and having a fixed end and a free end; a second generally U-shaped strip of bimetal nested within the first strip and having the layer with the higher coefficient of thermal expansion of the inner surface thereof with one end secured to a first electrical terminal, said second strip having a thermal force somewhat less than that of the first strip, the free end of the second strip extending outwardly beyond the free end of the first strip and in physical contact therewith but electrically insulated therefrom; a first electrical contact secured to a second electrical terminal; and a second electrical contact carried on the free end of the second bimetal strip and projecting from the lower expansion layer thereof and movable toward and away from engagement with said first electrical contact, said nested U-shaped strips being positioned relative to the first contact so that the second contact is in engagement therewith at temperatures below a preselected level whereby upon current flow between said terminals and through said second strip exceeding a level that will heat the strips above the preselected temperature level the first strip will flex to cause initial separation of the contacts and the resulting termination of currEnt flow through said second strip will cause it to cool and flex rapidly to increase the contact separation thereby to effect rapid contact separation under overtemperature conditions of the thermal protector.
 2. A protector as set forth in claim 1 wherein the second strip has a higher resistivity than that of said first strip.
 3. A protector as set forth in claim 1 wherein a layer of electrical insulation is interposed between the inner surface of said first U-shaped strip and the outer surface of the second strip.
 4. A protector as set forth in claim 3 wherein the insulation layer comprises a coating of a polyimide synthetic resin.
 5. A protector as set forth in claim 3 wherein said insulative layer is applied to the inner face of said first strip and said second U-shaped strip engages said insulative layer at least partially along the length of said second U-shaped strip thereby to enhance heat transfer between said strips.
 6. A protector as set forth in claim 3 wherein the outer surface of the second U-shaped strip and the inner layer of the first U-shaped strip both engage said insulative layer for substantially the length of said first U-shaped strip.
 7. A protector as set forth in claim 1 wherein there is an insulating abutment on the inner surface of the free end of the first strip and the outer surface of the free end of the second strip bears thereagainst.
 8. A protector as set forth in claim 2 wherein the flexivity of the first strip substantially exceeds that of the second strip.
 9. A protector as set forth in claim 1 further comprising a conductive housing, said one end of said first U-shaped strip being fixed relative to the housing and said one end of said second U-shaped strip being secured to and in electrical contact with said housing, said housing constituting said first terminal.
 10. A protector as set forth in claim 9 further comprising a conductive cover closing said housing and insulative means for electrically insulating said housing from said cover, said cover having said first contact secured thereto and constituting said second terminal.
 11. A protector as set forth in claim 9 wherein said housing has a deformable portion for adjusting the positioning of the two nested U-shaped strips and first contact relative to the second contact variably to bias the contacts together at temperatures below said preselected level thereby to calibrate the protector to open the contacts at a desired preselected temperature.
 12. A protector as set forth in claim 10 wherein said housing and said cover each have means for ready connection to an electrical circuit.
 13. A protector as set forth in claim 11 wherein the fixed ends of said U-shaped strips are commonly welded to said housing at the same point and wherein said insulative means comprises a layer of polyimide synthetic resin applied to the inner surface of said first strip to electrically insulate said first strip from said second strip, said first strip being clear of said polyimide coating in the area where said strips are welded together.
 14. A protector as set forth in claim 3 wherein the insulation layer comprises high temperature resin.
 15. A protector as set forth in claim 1 further comprising a terminal pin, a glass envelope, into which said pin extends, said one end of said first U-shaped strip being fixed relative to the envelope and said one end of said second U-shaped strip being secured to and in electrical contact with said pin, said pin constituting said first terminal. 